In the pulp industry, different processes are used for producing a cellulosic pulp from ligno-cellulosic feedstocks, typically softwoods and hardwoods. Even if also mechanical treatments may be used, most diffused processes comprise a cooking treatment of the ligno-cellulosic feedstock with chemicals for solubilizing the majority of the lignin and the hemicellulose, thereby producing a cellulosic fiber suspension in liquid solution called “brown stock”. By means of washing treatments, the brown stock is separated in a stream comprising the cellulosic fibers and one or more effluent streams, comprising the spent cooking chemicals and the solubilized lignin and hemicellulose. The clean pulp (stock) can be bleached in the bleach plant or left unbleached, depending on the end use.
In the dissolving pulp process, which is used to produce high purity cellulose end-product, at least a portion of the hemicellulose of the ligno-cellulosic feedstock is solubilized by means of a hydro-thermal pre-hydrolysis process prior to treating the ligno-cellulosic feedstock depleted of the extracted hemicellulose with chemicals.
Examples of pre-hydrolysis processes may be found in U.S. Pat. No. 8,262,854 and EP2430233.
In U.S. Pat. No. 8,262,854 it is disclosed an improved method for treating lignocellulosic material, including a prehydrolysis-mass transfer process, which produces a concentrated hydrolysate volume during the time required for the hydrolysis itself. The improved process comprises the heating of the digester and chip content by direct steam to the required hydrolysis temperature, starting a flow of hot, stored hydrolysate to the top of the chip bed in order to create a trickle-bed type down-flow of hydrolysate, collecting a first fraction of the trickled-down hydrolysate as a product fraction, adding extraction liquid and continuing the trickle flow to collect a second hydrolysate fraction, which will be discharged from the digester to a hot hydrolysate storage tank to be used as the first trickle flow liquid in the next batch.
In EP2430233 it is disclosed a displacement batch cooking process comprising a steam-phase prehydrolysis step, wherein the recovery of the by-products is improved. In the recovery step of the present invention, after the target P-factor in the prehydrolysis stage is reached, hot washing liquid is introduced into the digester from the bottom thereof. The washing liquid is circulated via the suction screens to the top and to the bottom of the digester until the prehydrolyzed chips are under the washing liquid. The hot washing liquid containing by-products is recovered from the digester and the digester contents are neutralized by displacing the washing liquid with alkaline liquor.
A common feature to all the processes in the pulp industry is the fact that they are very energy intensive. The effluent streams are usually sent to a recovery boiler, wherein they are burned generating heat from the solubilized lignin and hemicellulose. In the recovery boiler, chemicals are also recovered from the effluent stream and recycled in the process. As heat is a low value product, the conventional pulping processes only take a partial advantage from the ligno-cellulosic feedstock. Considering that hemicellulose has a low heating value, about half of that of lignin, burning hemicellulose to produce heat is not a convenient strategy for hemicellulose valorization. Moreover, the effluent streams, coming from the washing steps, are diluted and they must be subjected to many evaporation steps to increase the dry matter content before being burned, at the expenses of the energy balance.
The conversion of monomeric sugars to bio-chemicals or a bio-product is also known in the art. In this application is bio-chemicals or bio product referred to as any chemical from the groups of polyols, diols, alcohols, and carboxylic acids, or lactic acid or the group consisting of ethylene glycol, propylene glycol, or a mixture thereof. I.e. conversion products obtainable from monomeric sugars. U.S. Pat. No. 8,198,486 discloses methods for generating propylene glycol, ethylene glycol and other polyols, diols, ketones, aldehydes, carboxylic acids and alcohols from biomass. The methods involve reacting a portion of an aqueous stream of a biomass feedstock solution over a catalyst under aqueous phase reforming conditions to produce hydrogen, and then reacting the hydrogen and the aqueous feedstock solution over a catalyst to produce a generic mixture of propylene glycol, ethylene glycol and the other polyols, diols, ketones, aldehydes, carboxylic acids and alcohols.
US20110312051 disclosed a process for generating at least one polyol from a feedstock comprising saccharide performed in a continuous or batch manner. The process involves contacting hydrogen, water, and a feedstock comprising saccharide, with a catalyst system to generate an effluent stream comprising at least one polyol and recovering the polyol from the effluent stream. The polyol may be selected from the group consisting of ethylene glycol and propylene glycol.
In the biorefinery concept, different integrated processes for converting hemicellulose to various value-added products, such as polyols, alcohols, carboxylic acids and many others have been proposed.
As an example, in the process described in Mao H. et al., “Technical economic evaluation of a hardwood biorefinery using the “near-neutral” hemicellulose pre-extraction process”, J. Biobased Mater. Bioenergy 2(2) p. 177-185 (2008), a portion of the hemicellulose is extracted from wood prior to pulping and converted into acetic acid and ethanol while using the extracted wood chips to produce Kraft pulp. In the paper, an existing Kraft pulp mill was considered as the base case. The pulp production was maintained constant and the hemicellulose extraction process was added to the fiber line. The hemicellulose extraction process occurs in a separated impregnation vessel prior to the continuous digester for pulp production. The extraction is carried out using green liquor (mostly Na2CO3+Na2S). The process disclosed for hemicellulose extraction and conversion to ethanol and acetic acid includes wood extraction for hemicellulose removal, flashing of the extract to produce preheating steam, recycling a portion of the extract back to the extraction vessel for the purpose of raising the solids content of the extract, acid hydrolysis using sulfuric acid for conversion of the oligomeric carbohydrates into monomeric sugars and cleavage of lignin-carbohydrate covalent bonds, filtration to remove precipitated lignin, liquid-liquid extraction followed by distillation to remove acetic acid and furfural from the sugar solution, liming to raise the pH to that required for fermentation, fermentation of five and six-carbon sugars and glucuronic acid to ethanol and finally distillation and upgrading the product to pure ethanol.
There are many issues to be addressed or improved in the integration of pulping processes and hemicellulose conversion processes to value-added products, in order to render the integration effective from a technical or economical point of view.
One issue is related to the increase of hemicellulose extraction in pre-hydrolysis step with a minimal use of added chemicals, such as mineral acids, which are expensive and must be eliminated or recycled in downstream process steps.
As the pre-hydrolysis step produces water soluble hemicellulose mainly in the form of water soluble oligomeric and polymeric sugars, there is the need to hydrolyze the water soluble hemicellulose to monomers, which are then converted to the final product. It is known in the art that the hydrolysis of water soluble hemicellulose to monomers requires more severe process conditions than the pre-hydrolysis step, which may be reached by increasing hydrolysis temperature and/or time, and/or lowering the pH during hydrolysis, with respect to the pre-hydrolysis step. By increasing the hydrolysis time, the volume of the hydrolysis reactor may increase correspondingly to a not manageable size. By increasing the hydrolysis temperature, sugar degradation becomes relevant. As already stated, the use of added mineral acid to lower the pH should be minimized. Thereby, a second issue to be addressed is related to the integration of pre-hydrolysis and hydrolysis step, to obtain a whole process working in balanced process conditions.
A third issue is the production of pre-hydrolysis and hydrolysis streams having a high concentration of water soluble sugars, in polymeric and monomeric form.
All these open issues are solved by the disclosed invention.